With a few vigorously applied sweeps of a brush, the backdrop to Japan's best-known landmark is taking shape. A uniformly blue sky, a cobalt sea, and islands topped with lush pines.

But when it comes to sketching the familiar outline of the mural's centrepiece, Mizuki Tanaka must make way for her master. The 28-year-old has spent six years working alongside the veteran artist Morio Nakajima, but still has no idea when she might graduate to the select band of Tokyo artists who are allowed to paint Mount Fuji murals on the walls of the city's public baths.

Today they are painting an 18-metre long mural overlooking the bathing area at Moto no Yu, a 38-year-old public baths, or sento, in eastern Tokyo. The owners have paid about 100,000 yen (£770) for a work designed to induce total relaxation in bathers who come to observe the time-honoured ritual of soaking in frighteningly hot water.

The artists complete the half of the painting overlooking the women's baths in a few hours, before moving over to the men's side, separated by a ceramic wall just tall enough to frustrate potential voyeurs.

But despite a history stretching back 100 years, sento murals are at risk of dying out along with the tradition of hadaka no tsukiai, or naked friendship, as more urban Japanese choose to bathe at home. At their peak in the late 1960s, there were more than 2,500 sento in Tokyo; now there are fewer than 1,000.

"In the postwar period it wasn't unusual for hundreds of people a day, including entire families, to come here," says Moto no Yu's 83-year-old owner, Keiji Takeuchi. "Now we rarely reach 100, even on a busy day."

The decline of bathhouse culture means there is less work for Nakajima, who along with Kiyoto Maruyama is one of only two fully qualified Fuji painters. Toshimitsu Hayakawa died three years ago after a career spanning more than five decades.

Tanaka first became aware of Fuji murals while studying art history at Meiji Gakuin University. "I was blown away by it," she says during a rare break, "and decided I wanted to learn more."

When Nakajima completed his first Mount Fuji 47 years ago, he was one of eight specialist painters. Despite the decline in numbers, he was dismissive when Tanaka approached him about an apprenticeship.

"I refused to take her on at first," he says. "No woman had ever done this job before. It's hard work You spend a lot of time going up and down ladders with heavy pots of paint. But now I'm glad I gave in."

Less than 24 hours after the last application of oil paint, bathers still loyal to Moto no Yu will be greeted by a freshly painted Fuji as envisaged by Nakajima, who has painted thousands of Fuji murals, which are generally replaced every three years.

The scene is loosely based on the view of Yamanashi prefecture, but the inclusion of islands in the foreground is pure artistic licence.

"The scene doesn't have to be totally true to life," says Hiromi Kawaguchi, a self-proclaimed "sento freak" who has visited 2,500 bathhouses around Japan.

"You could say some are inspired by Hokusai [the Edo-era artist best known for his series of woodblock prints, Thirty-six Views of Mount Fuji]. Nakajima-sensei even does a deep red Fuji, but this bright version is his favourite."

There are as many views of Fuji as there are murals of the mountain, an active volcano and harbinger of good luck according to Japanese folklore. Nakajima's latest painting is lighter than the one it is slowly replacing, and each of the 70 or so others he paints in greater Tokyo over the course of the year differs in style and mood.

"Fuji is a really difficult mountain to paint, no matter how many times you do it," he says. "I think long and hard about each one. With this one I wanted to take advantage of the space. I want people to feel like they are taking a bath outside."

Every half an hour or so, he retreats to ground level, lights a cigarette and watches his apprentice at work. His advice is usually dispensed benevolently – more green here, bolder brush strokes there – but he grimaces when Tanaka is too liberal with the bright yellow.

Tanaka acknowledges it could be some time before her teacher allows her to paint the mountain itself. But that day will have to come if their art is to survive beyond his retirement.

"I could find myself alone in about 20 years so I'll do all I can to prevent that from happening. I'm determined to carry on and, one day, find my own apprentice."